Bridge Over The River Churn
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river Churn appears at Seven Springs in Gloucestershire, and runs down
through North Cerney, Baunton and the edge of Cirencester, finally joining the river Thames near South
Cerney. Its total length is only about 25 km, and at the junction,
the Thames is only about 13 km from its own source. At this point,
both rivers are very small.
Until recently, the river Churn was seen mainly by local people, and by those travelling between Cheltenham and Cirencester. Then came the Cirencester bypass, part of a scheme to improve the roads between the M4 and the M5. The improvement was very significant, cutting fifteen to twenty minutes from the journey time.
The page on appearance explains a few of many measures taken to mimimize the effects of this road on the landscape. One effect was to divert some parts of the road from the Roman alignment that had been in use for almost 2000 years. Cirencester is built on the site of the Roman settlement called Corinium, a name which is echoed both in the name of the town and in the name of river. Many bridges had to be built for this project.
The largest bridge crosses the valley of the river Churn. At the chosen crossing point, the valley is about forty metres deep. By cutting into the ground on either side of the valley, the road was able to reach the valley at a reasonable height, without steep approaches. The picture below shows a part of both the cuttings. The bridge is very slightly curved, both vertically and horizontally.
The next pictures are a view from the valley, and a horizontally squashed version, showing the slight curvature of the bridge. For this sort of thing you have to be sure that your lens does not have detectable barrel distortion or pincushion distortion. The cows represent one of the problems of outdoor photography. Though completely harmless, within a few more seconds they would have made the picture impossible by coming up very close, which is what they did.
Since cows are naturally curious, they will often approach, and if you time it right, you can use them to provide scale and interest to the picture.
There are several details in this picture. You can see the slight horizontal curvature of the bridge, which, together with the slight vertical curvature, gives such continuity of the road alignment that motorists may be barely aware that they are on a bridge. The piers are narrow at the bottom and wide at the top, where they support the bridge. Imagine the appearance if they were wide all the way, or if there were many individual narrow ones. Supporting a multi-span bridge in a tidy manner is a problem that has no easy solution. It is discussed in Leonhardt's book "Bridges". The bridge is an excellent example of a multi-beam bridge using continuous I-beams, which are made wider near the supports. Each beam has its own support on the pier. The beams are tied together with sloping ties for rigidity. Note the slight inward slope of the piers at the top. Why do think that was done?
The next picture shows how the support from each pier is carried up through the I-beam by an extra box section.
A few hundred yards downstream from this bridge, the Churn has to narrow from two or three metres wide to one metre wide, to pass under this little arch bridge that carries a farm track. The wide valley floor is a flat alluvial plain, across which the tiny stream wanders from side to side.
A couple of km further downstream, the Churn wanders through an old water meadow, which in wet winter times is traversed by a number of temporary streams, which are crossed by a number of small stone and concrete beams for farmers' use. Other such bridges have not survived. The pictures below show a few of these tiny bridges. The first picture includes a compressed inset showing that concrete, like any other material, can bend.
From here, the Churn flows into the lovely Cotswold town of Cirencester, and a few miles later, it joins the river Thames near the Cotswold Water Park, a series of lakes formed as a result of gravel digging. The Churn is one of many delightful small rivers, such as the Coln and the Windrush, which flow down the dip slopes of the Cotswolds, all eventually joining the Thames. Curiously, of all these rivers, the upper Thames is by no means the longest, and is certainly not the most distinguished.
Here are more pictures of the big bridge, showing the effects of different relative positions of the bridge, the sun, and the photographer, and of different weather conditions.
Just south east of this bridge, a minor road, the Whiteway, crosses the bypass. At this point, as on the other side of the Churn valley, the A417 is in a cutting, to reduce the gradient down to the Churn bridge, and to reduce the height of that bridge. The typical bridge in this situation would have been a simple concrete beam or cantilever type, but what we see is apparently a flat arch, faced attractively with Cotswold stone. But as we pass underneath, we see that the arch is only a facade, and that the bridge is a concrete beam. In fact, under the right hand side of the bridge, you can see a part of the sloping haunch.
Is it a fraud? Well, it looks a lot better in that location than a grey concrete bridge would have done. It avoids the concrete piers that would have blocked the view of the cutting, and it generates a lot less thrust than a real flat arch would have produced.